U.S. patent number 5,386,111 [Application Number 08/134,407] was granted by the patent office on 1995-01-31 for optical detection of water droplets using light refraction with a mask to prevent detection of unrefracted light.
Invention is credited to H. Allen Zimmerman.
United States Patent |
5,386,111 |
Zimmerman |
January 31, 1995 |
**Please see images for:
( Certificate of Correction ) ** |
Optical detection of water droplets using light refraction with a
mask to prevent detection of unrefracted light
Abstract
An optical detection system for detecting rain or other water
droplets on the outer surface of a window and fog on the inner
surface with a single photo-detector. The invention measures the
accumulation of water droplets on the window by light refraction of
a first light beam with droplets to redirect a first light beam to
the photo-detector. A masking device prevents light from reaching
the photo-detector directly without refraction. As a result the
rain measurement output signal of the photo-detector increases with
an increasing accumulation of water droplets on the window. The fog
accumulation is measured by a second light beam reflected off the
inner surface of the window to the photo-detector so that the
output signal of the photo-detector decreases with increasing
amounts of fog since fog scatters the light to reduce the amount of
light reflected to the photo-detector. A third light source is
focused directly on the photo-detector to bias it into an operating
point of high sensitivity to infrared light and is connected in a
negative feedback circuit from the output of the photo-detector
amplifier. The photo-detector output is connected to a narrowband
amplifier that is tuned to the frequency of an oscillator which
pulses the first and second light sources at different times. As a
result of this negative feedback, changes in the output signal due
to external factors are cancelled so they do not produce
errors.
Inventors: |
Zimmerman; H. Allen (Beaverton,
OR) |
Family
ID: |
22463244 |
Appl.
No.: |
08/134,407 |
Filed: |
October 8, 1993 |
Current U.S.
Class: |
250/227.25;
318/444 |
Current CPC
Class: |
B60S
1/0822 (20130101); B60S 1/0837 (20130101); G01N
21/41 (20130101); G01N 21/43 (20130101) |
Current International
Class: |
B60S
1/08 (20060101); G01N 21/41 (20060101); G01N
21/43 (20060101); H01J 005/16 () |
Field of
Search: |
;250/227.25
;318/483,444,DIG.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
"Tech Briefs", Automotive Engineering, Aug. 1991, p. 44..
|
Primary Examiner: Nelms; David C.
Assistant Examiner: Nichols; Steven L.
Attorney, Agent or Firm: Klarquist Sparkman Campbell Leigh
& Whinston
Claims
I claim:
1. An optical droplet detector apparatus for determining the degree
of vision impairment through a window due to an accumulation of
water or other precipitation on the window comprising:
a first light source for illuminating water droplets on said window
with a first light beam transmitted through said window to the
inside of said window;
a photo-detector located inside said window; and
a masking device positioned in front of said photo-detector to
effectively block the direct transmission of light rays from said
first light source to said photo-detector, but allowing said
photo-detector to receive light rays from said first source which
have been refracted by droplets on said window for the purpose of
determining the amount of water on said window so that the output
signal of said photo-detector increases with an increase in water
droplets on said window.
2. Apparatus in accordance with claim 1 wherein the first light
beam detects rain droplets on the outside surface of the window,
the masking device is positioned between the first light source and
the photo-detector and which also includes a second light source
positioned inside said window to produce a second light beam for
detecting fog on the inside surface of the window which reflects
off the inside surface to said photo-detector for the purpose of
determining the degree of fog accumulation on said inside surface,
said first and second light sources emitting infrared light.
3. Apparatus in accordance with claim 1 which also includes a third
light source positioned inside said window with its beam directly
aimed at said photo-detector for the purpose of optically driving
said photo-detector to its bias operating point and for providing
an optical feedback signal to said photo-detector.
4. Apparatus in accordance with claim 3, further comprising a drive
circuit for providing pulsed drive current to said first light
source for water accumulation measurements and another drive
circuit connected to said third light source for gain
stabilization, and an output circuit for amplifying the pulsating
output current produced by the photo-detector.
5. Apparatus in accordance with claim 4, wherein said output
circuit includes a narrowband amplifier and tank circuit closely
tuned to the frequency of the pulsed drive current for the purpose
of rejecting other, potentially interfering currents produced by
said photo-detector.
6. Apparatus in accordance with claim 4, wherein a feedback circuit
is connected from said output circuit to the drive circuit for the
third light source to provide an out-of-phase, negative feedback
signal to modulate said third light source to provide an optical
operational amplifier circuit for the purpose of overall gain
stabilization.
7. Apparatus in accordance with claim 1, further comprising
detector circuit means connected to the output of the
photo-detector for peak detection, averaging and temporary storage
of the photo-detector output signals to produce output signal data
pertaining to visibility impairment of said window, reducing the
effects of noise and unwanted signals in the data, and temporarily
storing the data for subsequent analog-to-digital conversion or
threshold comparison.
8. Apparatus in accordance with claim 2, further comprising an
analog-to-digital converter for converting the analog output signal
of the photo-detector pertaining to visibility impairment to a pair
of digital output signal related to measurements for rain and fog,
respectively, and computer interface circuits to interface said
digital output signal to a microprocessor for the purpose of making
threshold and state-transition decisions to control devices for
restoring visibility or for closing windows.
9. Apparatus in accordance with claim 8, wherein a digital offset
measurement is made of the output signals at a zero-drive-signal
condition obtained when neither said first light source nor said
second light source are driven for the purpose of subtracting said
offset value from the output signals corresponding to each
subsequent rain and fog measurement to eliminate any residual
zero-offset errors from said rain and fog measurements.
10. A water droplet detector for determining the degree of vision
impairment through a windshield of a vehicle due to an accumulation
of water on said windshield comprising:
a first sensor incorporating a first light source and a
photo-detector for the purpose of determining the degree of water
accumulation on the outside surface of said windshield;
a second sensor incorporating a second light source and said
photo-detector for the purpose of determining the degree of fog
accumulation on the inside surface of said windshield;
a third light source for the purpose of optically driving said
photo-detector to its bias operating point;
a drive circuit for providing pulsed drive current to said first
light source and to said second light source;
an output circuit connected to the photo-detector including a tuned
amplifier for providing narrowband amplification closely tuned to
the frequency of said pulsating drive currents for the purposes of
amplifying the pulsating output current produced by said
photo-detector and rejecting other, potentially interfering
currents produced by said photo-detector; and
a feedback circuit connected from said output circuit to said third
light source for providing attenuation and additional phase shift
to the output signal of said output circuit to provide a negative
feedback signal to modulate said third light source for the purpose
of gain stabilization of said output circuit.
11. Apparatus in accordance with claim 10, further comprising a
detector circuit means connected to the output circuit for peak
detection, averaging and temporary storage of the output signals
for producing output signal data pertaining to visibility
impairment of said windshield, for reducing the effects of noise
and unwanted signals in the data, and for temporarily storing the
data for subsequent analog-to-digital conversion or threshold
comparison.
12. Apparatus in accordance with claim 10, further comprising an
analog-to-digital converter for converting the analog output
signals of the output circuit pertaining to visibility impairment
to a pair of digital output signals relating to measurements for
rain and fog, respectively, computer interface circuits to
interface said digital output signals to a microprocessor for the
purpose of making threshold and state-transition decisions to
control devices for restoring visibility or for closing windows,
and offset means for measuring the digital offset of the output
signals at a zero-drive-signal condition obtained when neither said
first light source nor said second light source are driven, for the
purpose of subtracting said offset value from the output signals
corresponding to each subsequent rain and fog measurement to
eliminate any residual zero-offset errors from said rain and fog
measurements.
13. Apparatus in accordance with claim 10 in which the light
sources and their drive circuits together with the photo-detector
and its output circuit and feedback circuit are connected as an
optical operational amplifier for gain stability.
14. A method of optical detection of water or other precipitation
on a window to determine the degree of vision impairment through
the window due to the accumulation of water on said window,
comprising the steps of:
transmitting a first light beam from a first light source through
said window to the inside of said window;
detecting the first light beam with a photo-detector inside said
window;
blocking the direct transmission of light rays from said first
light source to said photo-detector by a light masking device;
and
refracting said first light beam with water droplets or other
precipitation on said window to cause a portion of said first light
beam to be redirected from said water droplets to said
photo-detector so that the detector output signal of said
photo-detector increases with an increase in water droplets on said
window.
15. A method in accordance with claim 14 in which the first light
beam is transmitted from a first light source outside the window
and is refracted by rain droplets on the outside surface of the
window, and which also includes the step of:
transmitting a second light beam from a second light source inside
the window so that said second light beam is reflected off the
inside surface of the window to the photo-detector for determining
the amount of fog accumulation on said inside surface by decreases
in the detector output signal with increases in fog on the inner
surface of said window due to diffusion of the light beam by the
fog.
16. A method in accordance with claim 14 which also includes the
step of:
transmitting a third light beam from a third light source directly
to the photo-detector to optically drive the photo-detector to its
bias operating point.
17. A method in accordance with claim 15 which also includes the
step of:
applying pulsed drive current selectively to the first light source
and to the second light source to cause them to emit light which is
pulsed at a predetermined frequency to cause the photo-detector to
produce a pulsed output signal.
18. A method in accordance with claim 17 which also includes the
step of:
transmitting the pulsed output signal of the photo-detector through
an output circuit including a narrowband amplifier and tank circuit
tuned to the frequency of the pulsed drive current.
19. A method in accordance with claim 18 which also includes the
step of:
transmitting a negative feedback signal from the photo-detector
output circuit to the drive circuit of the third light source.
20. A method in accordance with claim 18 which also includes the
steps of:
peak detection of the analog photo-detector output signal at the
output of the photo-detector output circuit;
converting the analog photo-detector output signal to a digital
output signal; and
processing said digital output signal with a computer to produce
digital data corresponding to measurements of the amount of water
on the window.
Description
The subject matter of the present invention relates generally to
optical detection of water and in particular to optical detection
of water droplets on a window, such as the windshield of an
automobile or other vehicle using light refraction. A light beam is
transmitted through the water droplets which refract the light beam
to a photo-detector which produces an electrical measurement signal
at the output of the photo-detector corresponding to the amount of
water accumulation on the window. The apparatus and method of the
present invention is especially useful for detecting moisture,
including rain drops and fog or other precipitation on the
windshield of automobiles or other vehicles in order to operate
windshield wipers, heaters, fans and other devices for removing
such rain and fog to improve the visibility through such windshield
or for operating other devices such as motors for closing
convertible tops, sunroofs, or other windows in the event of rain
when the vehicle is left unattended.
BACKGROUND OF THE INVENTION
It has previously been proposed in U.S. Pat. No. 4,131,834 to
Blaszkowski, issued Dec. 26, 1978, to provide moisture detectors
based upon measuring changes in electrical conductivity between
spaced electrodes which sense rain when the gap between such
electrodes is bridged by the rainwater. However, the amount of
conductivity varies with atmospheric contaminants in the water as
well as corrosion and wear of the electrical contacts forming the
electrodes. Therefore such moisture detectors do not provide
accurate measurement of the amount of moisture present.
In addition, moisture detectors have been proposed for detecting
moisture based on measuring the changing capacitance in the gap
between spaced electrodes due to changes in the dielectric material
of such gap, such as when water is present. However, such a
moisture detector suffers from poor sensitivity due to the
proximity effects of moving wiper blades on such capacitance and
from interfering electrical fields from power lines and other
sources.
It has also been proposed to detect moisture by sensing the sound
created by infringing droplets but this is inaccurate and is unable
to detect light mists or fog accumulations. Similarly, moisture
detectors based upon measurement of the mass changes due to the
presence of water droplets are insensitive to light mist or
fog.
Some optical detectors have sensed moisture based upon the
interruption of light beam by the water droplets. However, these
detectors also are insensitive to gradual accumulations of moisture
as mist or fog. Also, windshield wipers interrupt the light beam
and require gating mechanisms to disable the light detector during
wiper sweeps so they are somewhat impractical.
It is believed that moisture detectors which sense water droplets
by light refraction within the droplets are a substantial
improvement over these moisture detectors. However, previously
optical detectors which detect raindrops based upon light
refraction have suffered from several disadvantages, including
small detecting area, low sensitivity, error signals due to ambient
light, and dependence upon long-term stability of light sources and
photo-detectors whose characteristics change significantly with
temperature, aging, operating point, and supply voltage
variations.
The optical detection method and apparatus of the present invention
overcomes these problems using a first light beam transmitted
through a large area of the window and by employing a mask which
prevents the first light beam from directly reaching the
photo-detector unless such light beam is refracted by water
droplets on the outer surface of the windshield. As a result, the
output signal of the photo-detector indicating the presence of
water droplets is zero when no droplets are present and increases
in amplitude with the size and amount of water droplets present on
the windshield of the vehicle or other window for a more accurate
and more sensitive measurement of the accumulation of rain on such
window.
A second light source may be provided for measuring fog by
reflecting a second light beam off the inner surface of the window
to the photo-detector in order to detect fog on such inner surface.
As a result of diffusion of the second light beam by the fog less
light is reflected off of the window to the photo-detector so that
the fog measurement signal decreases in amplitude with increasing
amounts of fog. The output signal of the photo-detector for
measuring the accumulation of fog is distinguished from that for
measuring the accumulation of rain by operating the two light
sources at different times such as by electronically switching the
inputs of two current amplifiers driving such light sources in an
alternating manner to the output of a single oscillator. A third
light source directly radiates light upon the photo-detector to
bias it to the proper operating point. A narrowband amplifier tuned
to the oscillator frequency is connected to the output of the
photo-detector transistor to amplify the rain and fog measurement
signals. The output of such amplifier is connected through a
negative feedback circuit to the third light source to cancel gain
changes produced by changes in ambient light, temperature changes
and aging of the light source and photo-transistor, and power
supply variations.
It has been previously proposed in U.S. Pat. No. 5,059,877 to
Teder, issued Oct. 22, 1991, to operate a windshield wiper on an
automobile automatically by the optical detection of water droplets
on the windshield using light reflection from the outer surface of
the windshield. An accumulation of raindrops on such outer surface
scatters or diffuses the light beam and reduces the output signal
of the photo-detector with increases in raindrop accumulation. The
photo-detector is a photo-transistor which is coupled to the
windshield by a light pipe of small diameter which greatly reduces
the measured area of the windshield to less than approximately 1
sq. cm. This reduces the sensitivity of measurement, especially to
a small accumulation of raindrops. The optical detector system of
the present invention solves these problems by using light
refraction with a masking device in front of the photo-detector and
a wider light beam which covers a much larger area of the
windshield, over 31 sq. cm. This larger measurement area greatly
improves the accuracy of measurement of the amount of accumulated
rainfall. Also, the present invention operates in a more efficient
manner by refracting the light beam with the water droplets to
redirect it toward the photo-detector which is shielded from direct
radiation of such light beam by the masking device. As a result the
output signal of the photo-detector increases with an increase in
the amount of raindrops thereby improving its sensitivity. In
addition, the Teder rain measurement system is more sensitive to
changes in ambient light levels and therefore requires that a
compensation circuit sample and store the ambient light level
signals for subtraction from the measurement signal. Also, high
ambient light levels including bright sunlight or at night when the
headlights of an approaching car strike the windshield at a light
intensity greater than predetermined limits cause the raindrop
detection and wiper operation process to be suspended temporarily.
This ambient light problem is avoided in the optical detector of
the present invention by employing oscillator pulsed light sources,
a narrowband amplifier at the output of the photo-detector tuned to
the oscillator frequency and negative feedback from the output of
such amplifier through a bias light source directed at the
photo-detector.
U.S. Pat. No. 4,867,561 to Fujii et al., issued Sep. 19, 1989, also
shows a similar optical detector for detecting rain by light
reflection from the windshield in a detection area of extremely
small size of less than 2 sq. cm. The photo-detector is
two-dimensional array of photo-electric transducer elements mounted
within an optical system housing supported beneath the dashboard
closely adjacent the windshield. This optical detector employs
light reflection for sensing raindrops on the outer surface of the
windshield so that the presence of the raindrops reduces the amount
of light which is reflected to the photo-detector and thereby
reduces the output signal of such photo-detector. As a result the
Fujii detector system has limited sensitivity and reduced accuracy
compared to that of the present invention. Ambient light level
changes are also a problem with this detector. Thus the ambient
light level is measured and used to reduce the threshold levels of
the comparators in the detection circuit for measuring rain and fog
in an attempt to reduce inaccuracies due to change in the ambient
light level. Also no measurements may be made if excessive ambient
light is present such as bright sunlight.
A similar teaching is also shown in U.S. Pat. No. 4,595,866 to
Fukatsu et al., issued Jun. 17, 1986, which relates to an optical
detector for detecting rain on the windshield by the transmission
of light from an external light source outside the windshield to a
photo-detector within the automobile. The light beam is transmitted
directly to the photo-detector, so that the output signal of the
photo-detector is reduced when raindrops accumulate on the outer
surface of the windshield because they refract the light beam away
from such photo-detector. The present invention differs by
providing a mask in front of the photo-detector to prevent light
from being transmitted directly from the light source to the
photo-detector and refracting a portion of the light beam with the
detected raindrops to the photo-detector. As a result the output
signal of the photo-detector increases with increasing amounts of
raindrops on the windshield. The light detector of Fukatsu et al.
consists of a plurality of pairs of photo-detectors, each
photo-detector of a pair being positioned behind either an infrared
transparent strip or an infrared opaque strip with the outputs of
said pair of photo-detectors being connected to a differential
amplifier to measure the amount of rain accumulating on the
windshield. This optical detector is more complicated, expensive
and bulky. Also, it suffers from the problem of ambient light
because changes in ambient light would effect the output signals of
both photo-detectors of each pair. Finally, there is no way of
differentiating from the light detection of raindrops on the
outside surface of the windshield and the detection of fog on the
inner surface of the windshield.
The optical detection method and apparatus of the present invention
has several advantages over the above-discussed prior art,
including the ability to monitor a much larger area of rainfall on
the windshield so that the output signal of the photo-detector is
more accurate in measuring small accumulations of randomly located
droplets. In addition, by employing a mask to block light from
being directly transmitted from the light source to the
photo-detector and by employing light refraction from the raindrops
to redirect the light to the photo-detector, the output signal of
the photo-detector increases with increasing amounts of rain to
provide more sensitive detection at the onset of rain. Also, the
photo detection method and apparatus of the present invention is
capable of detecting small amounts of rain in the presence of high
ambient light and is not effected by changes in ambient light. The
optical detection method and apparatus of the present invention
also eliminates errors in the photo-detector output signal due to
external factors unrelated to moisture, such as changes in
temperature and aging of the LED light sources and photo-detector,
power supply voltage variations or changes in ambient light by
employing negative feedback through a reference light source. This
reference light source sets the bias of the photo-detector to an
operating point of high sensitivity to infrared light, and cancels
any changes in the photo-detector output signal due to these
external factors by negative feedback from the output of a tuned
amplifier connected to the photo-detector transistor through a
feedback circuit to the reference light source.
SUMMARY OF THE INVENTION
It is therefore one object of the present invention to provide an
improved moisture detection method and apparatus of high accuracy
and sensitivity in which water droplets on a window are detected by
optical detection using light refraction in the droplets.
Another object of the present invention is to provide such a
moisture detection method and apparatus of high sensitivity in
which a light masking device is positioned in front of a
photo-detector to prevent its direct irradiation by a light beam
which is refracted by such water droplets to the light detector to
produce a measurement output signal that increases in amplitude
with increasing amounts of moisture to measure the amount of
moisture accumulation on the window.
A further object of the invention is to provide such a moisture
detection method and apparatus using light refraction of a first
light beam for measuring the presence of rain or other water
droplets on the outer surface of a window and which employs a
second light beam for reflecting light off the inner surface of
such window to the same photo-detector in order to detect fog on
such inner surface and distinguishes between rain and fog
measurements by selectively switching between such first and second
light beams.
An additional object of the present invention is to provide such an
improved moisture detection method and apparatus in which the same
photo-detector is used to detect the first light beam and the
second light beam for measuring raindrops and fog in an efficient
and accurate manner.
Still another object of the invention is to provide such a moisture
detection method and apparatus in which the area of the first light
beam which strikes the window for detection of water droplets on
the window is greatly increased in size to provide a more accurate
raindrop accumulation measurement signal.
A still further object of the invention is to provide such a
moisture detection method and apparatus in which a third light
source is employed to provide a reference light beam for
irradiating the photo-detector directly in order to bias the
photo-detector at a proper operating point of high sensitivity to
such light and whose bias current supply circuit is connected in a
negative feedback path from the output of the photo-detector
amplifier to the third light source to cancel changes in the output
signal due to external factors including temperature changes and
aging of the light source or photo-detector, supply voltage
variations, and ambient light changes.
A still additional object of the invention is to provide such a
moisture detection method and apparatus in which the first and
second light sources are connected to an oscillator for pulsing
such light sources to produce a pulsed output signal of the
photo-detector and for amplifying such output signals with a
narrowband amplifier having a tank circuit which is tuned to the
oscillation frequency to reject other potentially interfering error
signals which might be produced by the photo-detector.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the present invention will be
apparent from the following detailed description of certain
preferred embodiments thereof and from the attached drawings of
which:
FIG. 1 is a diagram of one embodiment of a moisture detection
system using the method and apparatus of the present invention
suitable for detecting water droplets on the outer surface of the
window by refraction of a first light beam with such droplets and
for detecting fog on the inner surface of the window by reflection
of a second light beam from such inner surface;
FIG. 2 is a schematic diagram of a second embodiment of the
moisture detector system of the present invention in which the mask
used in FIG. 1 for preventing light from being transmitted directly
to the photo-detector from the first light source used for
detecting water droplets is changed to a horizon-type mask which
blocks the lower portion of the first light beam;
FIG. 3 is a schematic diagram of a third embodiment of the moisture
detection system of the present invention which the first light
source for measuring the accumulation raindrops on the outer
surface of the windshield is moved to a position inside the window
and is directed so that its light beam is reflected off of an
external mirror positioned outside of the window before striking
the water droplets and being refracted by such droplets to the
photo-detector, such internal light source acting as the masking
device to prevent light from such first light source from reaching
the photo-detector directly without being refracted;
FIG. 4 is a side view of the preferred embodiment of the moisture
measurement apparatus of the present invention used in a
measurement system in accordance with a modification of the system
of FIG. 1;
FIG. 5 is a plan view of the moisture measurement apparatus of FIG.
4;
FIG. 6 is a block diagram of the electrical circuit used for the
moisture measurement systems of FIGS. 1-4; and
FIG. 7 is an electrical circuit of a portion of the block diagram
of FIG. 6 .
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
As shown in FIG. 1, one embodiment of the moisture measurement
system of the present invention includes a first light source 10
positioned outside a light transparent window 12 such as the
windshield of an automobile or other vehicle. The first light
source 10 may be mounted under or on top of the automobile hood and
spaced from a photo-electric detector 14 mounted inside of such
window. A lens 16 in front of the first light source focuses the
light into a first light beam having a central axis 18 and a
conical shape such light beam being defined by an upper ray 20 and
a lower ray 22 and intersecting the window over a large area of
measurement. In this system the central axis 18 of the first light
beam is aligned with the photo-detector 14 which may be a
photo-transistor.
The first light source 10 is preferably a light emitting diode
(LED) which when energized emits a narrow beam of infrared light 24
that passes through the lens 16 and is focused by such lens into
the first light beam bounded by outer light rays 20, 22. While the
light beam may be of visible light it is preferably of infrared
light to avoid distraction of the vehicle driver.
In the embodiment of FIG. 1 a light opaque masking device 26, such
as a metal plate, is provided in front of photo-detector 14 and on
the axis 18 of the light beam to prevent the first light beam from
directly irradiating the photo-detector. Thus, the central viewing
axis 28 of the detector 14 through its lens 32 is aligned with the
beam axis 18 along a common axis and the mask 26 is positioned
across this common axis so that in absence of any water droplets
upon the window 12 the photo-detector 14 does not receive the first
light beam and produces substantially no output signal. However,
when a plurality of water droplets 30 accumulate on the outer
surface of the window 12 such water droplets refract the first
light beam and cause a portion of it to be redirected to the
photo-detector 14. As a result, an output signal is produced by the
photo-detector whose collector current amplitude is a measurement
of the amount of water droplets accumulated on the outer surface of
the window. The moisture measurement value corresponds to both the
number of water droplets and the area or size of such water
droplets within the measurement area on such window. Thus, the
upper beam limit ray 20 is refracted downward by the water droplet
and redirected as refracted light ray 20A. Similarly, the lower
beam limit ray 22 is refracted upward by a water droplet and
redirected as refracted light ray 22A. Both of the refracted rays
20A and 22A are focused by a lens 32 to the photo-detector 14.
A second light source 34 is provided inside the window to detect
fog or other moisture on the inner surface of the window 12. Thus,
the second light source 34 may be another light emitting diode
(LED) which emits infrared light to produce a second beam 36 that
is focused by a lens 38 on the inner surface of the window. This
second light beam is normally reflected off the inner surface of
the window 12 as reflected beam 36A directly to the photo-detector
14 to produce a fog measurement output signal. When fog or other
moisture is present on the inner surface of the window 12 a portion
of the second light beam 36 is scattered and diffused by the
moisture so that such portion is no longer reflected to the
photo-detector 14. As a result, the fog measurement signal produced
by the photo-detector 14 decreases in amplitude with greater
accumulations of fog on the inner surface of the windshield. It
should be noted that the second light source 34 is switched on at
different times than the first light source by an electronic switch
circuit in order to distinguish the fog measurement signal from the
rain measurement signal produced by the photo-detector 14 in a
manner hereafter described with respect to FIG. 6.
A third light source 40, such as an infrared LED, emits a third
light beam 42 which acts as a reference light beam and is
transmitted through a lens 44 directly to the photo-detector 14 in
order to bias the photo-detector at a preferred operating point on
its characteristic curve where it is of high sensitivity to
infrared or other light emitted by sources 10 and 34. In addition,
the third light source 40 may provide an optical negative feedback
connection for a circuit (not shown) from a photo-detector output
amplifier (not shown) to the photo-detector in a manner hereafter
described with respect to the circuit of FIG. 6 in order to
eliminate any changes in the measurement output signals of such
photo-detector due to external factors such as temperature changes,
aging of the light sources or photo-transistor, variations in the
power supply voltage, and ambient light changes.
As shown in FIG. 2, a second embodiment of the moisture detection
system of the present invention differs from that of FIG. 1 by
employing a horizon-type mask 46 which blocks the lower portion of
the first light beam 20, 22. The central viewing axis 28 of the
photo-detector 14 and its associated lens 32 is not in alignment
with the center axis 18 of the first light beam but is blocked by
the mask 46 so that substantially none of the first light beam
directly irradiates the photo-detector. However, when raindrops 30
accumulate on the outer surface of the window 12 they refract the
light beam so that the upper periphery ray 20 is refracted downward
as ray 20A to the photo-detector while the central axis ray 18 of
the first light beam is also refracted down as refracted ray 18A to
such photo-detector thereby causing an increase in the amplitude of
the rain measurement output signal of the photo-detector as a
measurement of the amount of raindrop accumulation on the window.
As a result of upwardly inclining the center axis 18 of the first
light beam so that it is not in alignment with the central viewing
axis 28, its bright center region is used to measure the rain
droplets which refract such light beam and redirect the refracted
beam ray 18A to the photo-detector. This improves the sensitivity
of the photo-detector to detecting raindrop accumulation. Other
than these changes, the second embodiment of FIG. 2 is similar to
that of FIG. 1 and the same reference numerals have been used in
FIG. 2 to designate like parts.
As shown in FIG. 3 a third embodiment of the moisture detection
system of the present invention differs from that of FIG. 1 by
positioning the first light source 10 and its associated lens 16 on
the inside of the window 12 and adding an external mirror 48
positioned outside of the window. As a result, the first light beam
20, 22 emitted by the first light source is focused by lens 16 and
transmitted through the window to the mirror 48 which reflects the
first light beam back through the window so that such beam is
refracted to the photo-detector 14 through its associated lens 32
when rain droplets 30 are present on the outer surface of the
window. However, when no raindrops are present on the outer surface
of window 12 the boundary rays 20A, 22B of the first light beam are
not redirected to the photo-detector but instead are redirected so
that they do not reach the photo-detector. It should be noted that
in FIG. 3 the light beam passes through the window twice and is
therefore attenuated more than that of FIG. 1 so that this system
is not as sensitive as FIG. 1. Also in FIG. 3 the first light
source 10 functions as a masking device in front of the
photo-detector 14 thereby replacing the mask 26 of FIG. 1 and
blocking the central viewing axis 28 of the photo-detector from
directly receiving any unrefracted light from the first light
source. The second light source 34 and the third light source 40
function in a similar manner in FIG. 3 to their corresponding
elements in FIG. 1 and will not be described further. It should be
noted that each of the three light sources 10, 34, 40 in all of the
embodiments of FIGS. 1-3 are preferably light emitting diodes (LED)
which emit light of the same wavelength, preferably infrared. Also,
the photo-detector 14 is preferably a photo-transistor which is
sensitive to infrared light.
A preferred embodiment of the moisture detection apparatus of the
present invention is shown in FIGS. 4 and 5 which provides a
modified version of the optical detection system shown in the
schematic diagram of FIG. 1. The apparatus includes first infrared
light source LED 10 and associated lens 16, second infrared light
source LED 34 and associated lens 38, an infrared photo-detector
transistor 14 and associated lens 32, and third infrared light
source LED 40 and associated lens 44. These three light sources and
the photo-detector and their lenses are all supported in a similar
manner to FIG. 1 to be properly positioned with respect to the
window 12 which may be the windshield of an automobile or other
vehicle. The first light source 10 and its lens 16 are supported on
top of or beneath a hood 50 of the automobile so that the central
axis 18 of the first light beam is inclined at an angle of about
13.degree. to a horizontal reference plane 52 and intersects the
windshield at point 53 but is not in alignment with the central
viewing axis 28 of the photo-detector 14. Instead, unlike FIG. 1
only a lower portion of the first beam, not the center axis 18, is
blocked by the mask 26 to prevent such lower portion of the first
light beam from being directly transmitted to the photo-detector. A
construction line 55 extending from the center of mask 26 to the
first light source is at an angle of 10.degree. with respect to the
horizontal reference plane 52 and at an angle of 3.degree. with
respect to central axis 18. As a result the central axis 18 of the
first light beam and its corresponding bright center pass above the
mask 26 and is refracted to the photo-transistor 14 by water
droplets on the outer surface of the windshield 12 to measure the
accumulation of raindrops with greater sensitivity.
The windshield 12 forms an angle of approximately 30.degree. with
the horizontal reference plane 52. The center viewing axis 28 of
the viewing field of the photo-transistor 14 and its associated
lens 32 intersects the center axis 18 of the first beam on the
outer surface of the windshield 12 at point 53. Viewing axis 28
makes an angle of 4.degree. with the construction line 55 through
the first light source 10, such angle extending above such line.
The center axis of the third light beam 42 also makes an angle of
about 4.degree. with respect to the construction line 55, such
angle extending below such line, and intersects the center of the
lens 32 of the photo-transistor. The second light source 34 emits
the second light beam 36 which forms an angle of incidence of
28.degree. with respect to the inner surface of the windshield 12
and the reflected second beam 36A forms an angle of reflection of
28.degree. with such inner surface of the windshield as it is
reflected to the photo-transistor. Also, the axis 36 of the second
light beam forms an angle of 44.degree. with respect to the axis 42
of the second light beam.
The second light source 34 and the third light source 40 are both
mounted on a metal support plate 54 which is connected by a swivel
joint 56 at one end of such plate to an L-shaped support bracket 58
welded to a flat support plate 60 which may be cemented to the
bottom of the windshield. The opposite end of the support plate 54
is secured by a screw 62 to a suitable support member 64 fixed to
the upper surface of the dashboard of the automobile. The
photo-detector 14 and its associated lens 32 are secured to the
upper surface of the opposite end of support plate 50 within a
tubular housing 66 welded to such plate and having an over-hanging
hood which shields the photo-detector from ambient light sources.
The third light source 40 is mounted within a first tubular member
68 which extends within a second tubular member 70 fixed to plate
54. The mask 26 is mounted on the top of the tubular member 70
which is of the proper inner diameter to receive the first tubular
member 68 and to hold it in a sliding fit enable the third light
beam of light source 40 to be transmitted therethrough to the
photo-detector. The second light source 34 is fixed by a bracket 72
welded to the top of the first tubular member 68 to enable
alignment of the center axis of the reflected second beam 36A with
the photo-detector 14 by pivoting the first tubular member within
the second fixed tubular member 70.
In the preferred embodiment of FIGS. 4 and 5 the first light source
10 is spaced a distance of about 6-3/8" from the windshield 12 at
intersection point 53 along its center axis 18. The photo-detector
14 is spaced a distance of about 5-7/16" along its central viewing
axis 28 from the windshield at the intersection point 53. The third
light source 40 is spaced along axis 42 a distance of 4-15/16" from
the lens 32 of photo-detector 14. It should be noted that the
cathode leads of the second light source 34 and the third light
source 40 may each have one terminal connected together to provide
thermal coupling for temperature compensation.
A moisture detection measurement circuit in accordance with the
present invention is shown in FIG. 6 and includes an oscillator 74
which produces a square wave output signal having a frequency of
approximately 37 Kilohertz. The oscillator output signal is
supplied through an electronic switch 76 having two output
terminals 75 and 77 connected respectively to the inputs of a pair
of current drive amplifiers 78 and 80 which drive the first light
source 10 and the second light source 34, respectively. Thus, the
output signal of the oscillator 74 is applied to a selected one of
the driver amplifiers 78 and 80 in accordance with the position of
the switch 76 so that one of the light sources 10 and 34 is pulsed
at a time to measure rain or fog by the same photo-detector 14 at
different times. In addition, a bias current source 82 is connected
to the third light source 40 and such current source is connected
to a source of DC bias voltage 84 which biases the third light
source normally on. The third light source 40 normally biases the
photo-detector transistor 14 to its proper operating point for high
sensitivity to infrared light. The output of the photo-detector 14
which may be a photo-transistor, is connected to a tuned amplifier
circuit 86 which includes an RC tank circuit tuned to the 37
Kilohertz frequency of the oscillator 74. The sine wave output
signal of the tuned amplifier 86 is transmitted through an RC phase
shifter circuit 87 forming part of a negative feedback circuit 88
which is connected from the output of amplifier 86 to the bias
current source 82 to amplitude modulate the third light source with
a negative feedback sine wave signal. Also, the third light source
40 may be thermally coupled to the second light source 34 such as
by connecting their cathode leads together for thermal
compensation. As a result of this negative feedback, any external
changes in the photo-transistor output signals due to power supply
variations, temperature changes or aging of the light sources 10
and 34 and the photo-transistor and changes in ambient light will
be cancelled by the negative feedback signal. In addition, the
drive amplifier 78 or 80, the light sources 10 or 34 and 40, the
photo-transistor 14, tuned amplifier 86 and the negative feedback
circuit 82, 87, 88 in effect form an optical operational amplifier
whose gain is determined by the values of the passive circuit
elements including the emitter resistor of the drive amplifier
transistor 78 and the resistors 140, 142 and capacitor 144 of the
phase shifter 87 in the feedback path 88 for better overall gain
stability.
Of course, the tuned amplifier and its associated tank circuit
change the square wave signal pulses produced by the
photo-transistor 14 in response to the light pulses of light
sources 10 and 34 into a sine wave voltage which is amplified. This
amplified sine wave is then peak detected and stored in a peak
averager and memory circuit 90. Two separate memories are employed
for storing the rain measurement signal, respectively, and the fog
measurement signal and they are selectively connected by an
electronic switch (not shown) to the output of such circuit. The
analog output signal of the peak averager and memory circuit 90 is
transmitted through a buffer amplifier 92 to one input 93 of a
voltage comparator 94.
A clock pulse generator 96 producing clock pulses having a
frequency of approximately 20 Hertz is connected at its output 97
to a start input of a staircase voltage generator 98 in order to
enable such staircase generator to start to produce a stair-step
voltage which increases one step for each output pulse of the
oscillator 74 whose output is also connected to the staircase
generator at a step input terminal 100. The stair-step voltage
generated at output terminal 102 of the staircase generator is
connected to a second input of the voltage comparator 94 so that
when such stair-step voltage exceeds the averaged peak measurement
analog voltage at the first input 93 of the comparator such
comparator switches to produce an output pulse at comparator output
104.
As stated the start input terminal of the gate 106 is connected to
the start output 97 of the clock 96 which starts the counter gate
and the staircase generator at the same time. The output pulse of
the comparator 94 is fed to the stop input terminal of a counter
gate 106 to turn off such gate. As a result, counter gate 106
transmits output pulses of the oscillator 74 through such gate to
the counter 108 for counting such oscillator pulses to produce a
digital output measurement signal at the output 110 of such counter
which corresponds to the measurement of the detected amount of rain
or fog which has accumulated on the windshield of the automobile.
This digital measurement output signal at output 110 is connected
through a computer interface circuit 112 to a conventional digital
computer, such as a microprocessor which uses the measurement value
to control the operation of moisture removal devices. A result
ready signal is applied by the output terminal 104 of the
comparator 94 to the computer interface 112 to enable it to process
the digital measurement signal produced at the output 110 of the
counter.
Alternatively, for moisture measurement in a non-automobile
application the digital output signal of the counter may be
transmitted from output 110 to a display segment decoder circuit
116 which decodes the digital signal and applies a corresponding
measurement signal to a three digit display circuit 118 which
displays the value of the moisture measurement. The clock 96
produces a reset signal which is applied to the counter 108 to
reset the counter to zero at the end of a measurement and a
blanking signal to the display segment decoder 116 to blank such
decoder between measurements.
It should be noted that the water accumulation measurement signal
at the output of the counter 108 is a measure of both the number
and size of the water droplets detected by the first light source
10 and the photo-transistor 14 and therefore represents the total
amount of water accumulated on the outer surface of the windshield.
Also the value of this measurement signal increases with an
increasing amount of water droplets on the outer surface of such
windshield. However, when fog is measured on the inner surface of
the windshield by the second light source 34 and the
photo-transistor 14 the output signal of the counter 108 decreases
with increasing amount of fog. This difference between the rain and
fog signals is taken into account when the signals are processed by
the computer 114 for a proper display of the measurement values of
rain and fog and proper operation of moisture removal devices by
control signals at the control outputs 120 of the computer.
Also the electronic switch 76 for switching the output of the
oscillator 74 to either the input 75 of the driver amplifier 78 of
the first light source 10 or the input 77 of the driver amplifier
80 of the second light source 34, is controlled by a control signal
generated by a separate control logic circuit or by the computer at
one of the control outputs 120 for alternately taking measurements
of the rain droplet accumulation on the outer surface of the
windshield or fog measurements of the amount of fog accumulation on
the inner surface of the windshield. The computer output control
signal is employed to operate various visibility improving devices
such as windshield wipers which may be turned on and whose speed
may be varied by the computer depending upon the raindrop
accumulation. Also electrical heaters and air blowers may be
operated to remove the fog from the inner surface of the windshield
of the automobile. In addition, the computer output control signal
can also be used to control motors for closing windows such as the
sunroof window of an automobile or raising the convertible top of a
convertible-type automobile.
As shown in FIG. 7, the tuned narrowband amplifier circuit 86 has
two amplifier stages including a first stage having a first LC tank
circuit 122 including a first transformer 124 with its primary
winding connected in series with the collector of the
photo-detector transistor 14 and having its secondary winding
connected in parallel with a capacitor 126 of the proper value so
that such tank circuit is tuned to the 37 Kilohertz frequency of
the oscillator 74. It should be noted that a switching transistor
127 is connected to the upper end of the primary winding of
transformer 124 to prevent the photo-transistor from producing a
measurement signal when such switching transistor is switched on to
produce a hold signal which disables the measurement clock 96, such
as when a high brightness ambient light drives the photo-transistor
into saturation. The output of the tank circuit 122 is connected to
the positive input of a first stage amplifier 128 through a
coupling resistor 129. The oscillator through the electronic switch
76 selectively applies the oscillator pulses to inputs 75 or 77 of
the driver amplifiers 78 or 80 for the first and second light
emitting diodes 10, or 34, respectively. It should be noted that
the input 77 is connected through a variable resistance
potentiometer 79 to the emitter of the driver amplifier transistor
80 in order to adjust the amplitude of the fog drive input signal,
and the base of such transistor is connected to the DC bias voltage
at terminal 84. The DC bias voltage sources indicated as "+5d" and
"+10d" are LC decoupled DC voltage sources of +5 volts and +10
volts.
The second stage of the tuned amplifier 86 includes a second tank
circuit 130 with a second transformer 132 having its primary
winding connected in series with a load resistor 134 to the output
of amplifier 128. The secondary winding of transformer 132 is
connected in parallel with a capacitor 136 of the proper value to
tune the second tank circuit 130 to the same 37 Kilohertz frequency
of the oscillator. The output of the second tank circuit is
connected through a coupling resistor 137 to the positive input of
a second amplifier 138 which produces an output signal voltage at
its output terminal 139.
A negative feedback circuit is connected from the output 139 of the
second amplifier 138 through an RC phase shift circuit 87 including
an input coupling resistor 140 an output coupling resistor 142 and
a shunt capacitor 144 connected from a point between such resistors
and ground.
The negative feedback signal is applied to the emitter of a
transistor 146 in the current supply circuit 82 which supplies bias
current for the third light source 40. The base of transistor 146
is connected to a source of DC bias voltage at terminal 84 which
normally biases such transistor conducting to cause current to flow
from the collector of such transistor through the light emitting
diode (LED) 40 to normally bias such LED on so that it emits the
third light beam. This third light beam is directed onto the
photo-transistor 14 in order to optically bias such
photo-transistor at an operating point on its characteristic curve
of high sensitivity to infrared light. It should be noted that the
driver amplifiers 78, 80 of the first light source 10 and second
light source 34 are normally biased off and are switched into an on
condition by the square wave oscillator signals applied to input
terminal 75 and 77 by the electronic switch 76 as shown in FIG. 6.
Thus, the light sources 10 and 34 are pulsed on and off by the
oscillator signal square wave pulses to produce a corresponding
pulsed output signal on the collector of the photo-transistor 14
which is then changed into a sine wave by the tuned tank circuits
122, 130.
The negative feedback signal from the third light emitting diode 40
is coupled by photo-transistor 14 and the primary winding of
transformer 124 to stimulate tank circuit 122 in a manner which is
180.degree. out of phase from the stimulation produced in tank
circuit 122 by the input signal from the first or second light
emitting diode 10 or 34. As a result, the tank circuit voltage is
reduced to a small fraction of what it would otherwise be with no
feedback signal applied. The effects of sensitivity changes in the
light emitting diodes or photo-transistor caused by temperature
changes, aging, power supply variations or changes in ambient light
are also reduced accordingly. Using tank circuit 122 as the
starting point, the voltage produced by the input signal from the
first or second light emitting diode 10 or 34 is phase shifted a
total of 22.degree. in the circuits associated with amplifier 128,
transformer 132, tank circuit 130 and amplifier 138. Phase shifted
circuit 87 adds 68.degree. while transistor 82, light emitting
diode 40 and photo-transistor 14 do not add appreciable phase
shift. The normal phase difference at resonance between the
inductor current and the capacitor voltage in tank circuit 122 adds
another 90.degree. for a total phase shift around the loop of
180.degree..
The sine wave output signal of the second amplifier stage 138 of
the narrowband amplifier 86 is transmitted from output terminal 139
to the input of the peak averager and memory circuit 90 where it is
averaged and stored as a DC analog voltage in either a rain memory
capacitor 148 or a fog memory capacitor 150. A first charging gate
including a first pair of anode connected gating diodes 152, 154 is
connected between the output of amplifier 138 and the upper plate
of rain memory capacitor 148 to charge such capacitor to the peak
amplitude of the rain measurement output signal only when such gate
is rendered conducting by a computer control square wave gate
signal applied to a gate terminal 156 connected to the common
connection of the anodes of such diodes. Switching transistor 158
is connected as a shunt to the +5 volts DC supply between rain
memory capacitor 148 and the memory output 172. During a rain
measurement, a square wave signal applied to control terminal 160
connected to the gate of field effect transistor 158 renders it
non-conducting such that the rain measurement signal stored in rain
memory capacitor 148 reaches the memory output 172. During a fog
measurement, however, transistor 158 is rendered conducting thus
inhibiting the stored rain measurement signal from reaching the
memory output 172. A similar charge gate 162, 164 and switching
transistor 168 are provided for the fog memory capacitor 150. Thus,
the fog memory capacitor 158 is connected through a second charging
gate formed by a pair of diodes 162 and 164 having their common
anode connection connected to a gate control input 166 for
rendering such gate conductive to charge the fog memory capacitor
150 from the output of the amplifier 138 through such gate.
Switching transistor 168 is connected as a shunt to the +5 volts DC
supply between fog memory capacitor 150 and the memory output 172.
During a fog measurement, a square wave signal applied to control
terminal 170 connected to the gate of field effect transistor 168
renders it non-conducting such that the fog measurement signal
stored in fog memory capacitor 150 reaches the memory output 172.
During a rain measurement, however, transistor 168 is rendered
conducting thus inhibiting the stored fog measurement signal from
reaching the memory output 172. It should be noted that the
charging control signals on terminals 156 and 166 are square waves
which are phase inverted with respect to each other so that gate
152, 154 is open when gate 162, 164 is closed and vice versa.
However, there is a time delay between the termination of the gate
on signal at terminal 156 and the start of the gate on signal at
terminal 166. During such time delay a charge voltage on the rain
memory capacitor 148 is transmitted through the buffer amplifier 92
to the comparator for operating the counter gate 106 to cause the
counter 108 to count the rain measurement in FIG. 6. The rain
measurement signal at counter output 110 is subsequently displayed
after the count is completed and while the fog signal is charging
fog memory capacitor 150.
The disabling control signals on terminals 160 and 170 are phase
inverted with respect to each other so that switch 158 is on while
switch 168 is off and vice versa. As a result, depending upon
whether switches 158 and 168 are on or off either the rain
measurement signal stored on memory capacitor 148 or the fog
measurement signal stored on memory capacitor 150 is supplied from
the output 172 of the memory through the buffer amplifier 92 to the
comparator 93 of FIG. 6. In this way, the moisture detection system
produces with light sources 10 and 34 at different times the two
output measurement signals at the output 110 of the counter 108
including a rain measure signal corresponding to the raindrop
accumulation on the outer surface of the windshield and a fog
measurement signal corresponding to the fog accumulation on the
inner surface of such windshield.
It should be noted that the charge control signals applied to
control terminals 156, 166 and the disabling control signals
applied to control terminals 160, 170 are all produced by the
computer and supplied from different ones of its control output
terminals 120 at appropriate times as is the control signal for
operating the electronic switch 76 for selecting light sources 10
and 34 which determines whether rain or fog measurements are to be
taken.
It should be noted that the above-described preferred embodiments
of the present invention are merely illustrative of the present
invention. Many changes may be made in such preferred embodiments
which will be obvious to those having ordinary skill in the art.
Therefore, the scope of the present invention should only be
determined by the following claims.
* * * * *